The present invention relates to coated abrasives and specifically to coated abrasives in which the abrasive particles are held in position by a UV-curable binder.
In the manufacture of coated abrasives, abrasive particles are usually adhered to a backing material by a maker coat and a size coat is placed over the abrasive particles to anchor them in place. Sometimes a supersize coat is applied over the size coat to impart some special property such as anti-loading, antistatic character or to place a grinding aid at the point at which the abrasive particles contact a work piece during use.
Binders most frequently used for the maker and size coats in such structures were and still are phenolic resins though other thermosetting resins have also been used at times. However such binders are slow to cure and require expensive drying and curing equipment to be effective. For this reason in part faster curing binders including those cured using UV radiation have been proposed and to some extent adopted.
As used herein it is understood that the term "UV-cured or UV-curable" embraces resins that can be cured by exposure to actinic light in the visible or ultraviolet part of the spectrum and to electron beam radiation.
Cure of such binder is accelerated by the use of one of a number of classes of photoinitiators which generate free radicals when exposed to UV light. These groups of free-radical generators include organic peroxides, azo compounds, quinones, benzophenones, nitroso compounds, acryl halides, hydrozones, mercapto compounds, pyrylium compounds, triacrylimidazoles, bisimidazoles, chloroalkyltriazines, benzoin ethers, benzil ketals, thioxanthones and acetophenones, including derivatives of such compounds. Among these the most commonly employed photoinitiators are the benzil ketals such as 2,2-dimethoxy-2-phenyl acetophenone (available from Ciba Specialty Chemicals under the trademark IRGACURE.RTM. 651) and acetophenone derivatives such as 2,2-diethoxyacetophenone ("DEAP", which is commercially available from First Chemical Corporation), 2-hydroxy-2-methyl-1-phenyl-propan-1-one ("HMPP", which is commercially available from Ciba Specialty Chemicals under the trademark DAROCUR.RTM. 1173), 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone, (which is commercially available from Ciba Specialty Chemicals under the trademark IRGACURE.RTM. 369); and 2-methyl-1-(4-(methylthio)phenyl)-2-morpholinopropan-1-one, (available from Ciba Specialty Chemicals under the trademark IRGACURE.RTM. 907).
With the assistance of such photoinitiators such resins cure essentially completely in minutes rather than hours and therefore afford the opportunity for significant cost saving. They do however have a drawback in that, in the presence of solid materials, the cure is often incomplete in areas shielded from the activating light. This can happen as the result of the incorporation of pigments or fillers but it can also happen in the absence of solid materials and merely because the resin layer is particularly thick.
The shielding effect is perhaps acceptable where the resin is applied over abrasive grains such that the greater bulk of the resin is exposed to the UV light during cure. However certain newer products depart from the maker/abrasive particles/size structure by adding the binder and the abrasive particles in the form of a mixture in which the cured binder both adheres the mixture to the substrate backing and acts as a matrix in which the abrasive particles are dispersed. This mixture may be deposited in the form of a uniform layer on the substrate or in the form of a pattern comprising a plurality of composites in repeating patterns, each composite comprising abrasive particles dispersed in the binder, to form the so-called structured or engineered abrasives. It will be appreciated that the shielding effect in such products is quite significantly greater and tends to limit the size of the abrasive particles that can be used and the thickness of the abrasive/binder layer that may be deposited on a substrate.
Incomplete cure is particularly disadvantageous in portions of the structure where the resin contacts the substrate since it leads to poor adhesion to the substrate and poor adhesion leads to poor grinding performance. However this is precisely where the effect is at its most pronounced because it is where the depth of cure and shielding effects are most pronounced.
A new group of photoinitiators has now been discovered to be surprisingly effective in curing UV-curable resins to greater depths than hitherto considered possible without the assistance of thermal cure initiators. This leads to the possibility that relatively large composites can form part of engineered abrasive products. It also makes possible the elimination of thermal initiators to complete cure of the resin.